They looked at 34 previous studies of over 2 million shiftworkers. Together, there were over 17,000 heart events, over 6000 heart attacks and almost 2000 strokes. They concluded that stroke risk was around 5% higher and coronary event risk 24% higher in the shift-workers. Surprisingly, despite their large dataset, the researchers found no associated increase in mortality.

They take care to point out, as with any other observational study, that this is not necessarily a cause and effect relationship. People who take on shift work may be different in some unconnected way that puts up risk of vascular disease. Lower socio-economic status is an obvious important confounder. The work itself may be more stressful, at the same time as their being fewer other staff members in support ‘out of hours’. Shift workers may also take more caffeine to try to stay alert during often long shifts.